1. Field of the Invention
This invention relates to a process for preventing the discoloration of paper and paper prevented from discoloring.
More particularly, it relates to a process for preventing the discoloration, in particular, caused by irradiation of paper with light which comprises adding or applying hypophosphorous acid or its salt to the paper at any step during or after papermaking, and to paper prevented from discoloring by adding or applying hypophosphorous acid or its salt thereto.
2. Description of the Prior Art
It is widely known that paper is made by mechanically or chemically treating a vegerable material such as wood to thereby give a pulp such as mechanical, chemical, semichemical, wastepaper, hemp or linter pulp and feeding said pulp into a paper machine.
When an unbleached pulp is fed to a paper machine as such, the obtained paper has an unsatisfactory whiteness. In such a case, the pulp may be bleached through oxidating by using, for example, chlorine, hypochlorites, chlorine dioxide, hydrogen peroxide or oxygen or through reduction by using, for example, hydrosulfite or aqueous sulfurous acid, if desired.
A papermaking process comprises a preparation step where a pulp or a mixture thereof is ground and chemicals such as a sizing agent or a filler are added thereto, a papermaking step where the above mixture is treated with various papermachines, dehydrated, dried and glazed, and a conversion and finishing step where a coating suitable for the purpose is applied onto the surface of the resulting paper. Thus a paper having the desired properties is obtained.
Although the paper thus obtained has a certain whiteness immediately after the production, it suffers from serious discoloration when exposed to sunlight involving UV light. Such a discoloration occurs regardless of the type of the pulp or bleaching.
Recently, the application of so-called high-yield pulps has been more and more increasing in order to efficiently utilize wood resources and to lower waste matters. These high-yield pulps contain a large amount of lignin and thus suffer from significant discoloration upon irradiation with light. This causes a serious problem when these high-yield pulps are employed not only alone but also as a mixture with chemical pulp(s).
It has been attempted to suppress the discoloration upon irradiation with light. For example, it is proposed to add an UV absorber to paper. However this method is disadvantageous in that a large amount of an expensive UV absorber is required and the UV absorber generally has a yellow color, thus imparting an undesirable color to the paper.
It was reported that a low molecular weight mercapto compound such as thioglycerol or thioglycol was effective in the prevention of the discoloration of high-yield pulps caused by light (cf. Tappi Journal, Nov. 1987, 117-122). However this method was inavailable in practice, since it could not give any satisfactory effect and, furthermore, the mercapto compound to be used had an offensive odor.
Thus it has been eagerly desired to establish a process for preventing the discoloration of paper without exerting any undesirable effects on other properties of the paper.
U.S.S.R. Patents No. 485178, No. 542775, No. 697617 and No. 857328 disclose that the use of a hypophosphite in the production of pulp enables the Production of the pulp at a high yield. However it is obvious that the hypophosphite used during the production of the pulp would never substantially remain in the paper. Furthermore, none of these patents discloses an effect of preventing the discoloration of the paper after the completion of the papermaking.